Electric cable



Patented oei. s, 1940 UNITED STATES PATENT OFFICE signor to CallendersCable and Construction Company Limited, London, England, a BritishCompany Application September 7, 1938, Serial No. 228,768 In GreatBritain September 28, 1937 6 Claims.

This invention relates to high tension electric cables comprising aconductor and a dielectric of paper or other iibrous materialimpregnated with oil, oil-compound or other insulating liquid,hereinafter for the sake of brevity termed compound. Cables of this typewhether the solid part of the dielectric is paper or other fibrousmaterial, will hereinafter be termed, for convenience, impregnated paperinsulated cables.

In the specification of prior Patent No.2,102,974 it has been indicatedthat breakdown of the dielectric of impregnated paper insulated cablesis in the great majority of cases initiated at or close to the conductorin spaces containing gas and having the conductor as the inner boundaryand the dielectric as the outer boundary.

In accordance with the present invention I provide a'breakdown-resistant cable by interposing between the surface of theconductor and the innermost layer of the impregnated body of dielectrica thin, and preferably continuous, composite layer comprising an innerportion consisting of an artificially formed lm of semiconductivematerial and an outer portion consisting of homogeneous insulatingmaterial impervious to the compound used to impregnate the dielectric.The two parts of the composite layer are as far as possible -in completecontact throughout their adjacent surfaces.

'I'he improved protective layer is to be distinguished from a protectivelayer of known form consisting of a lm or wrapping of homogeneousinsulating material applied directly to a conductor whose surfacecarries a naturally-formed film of oxide. Such naturally-formed oxidelms have a thickness of 1x10- p. or less and are much too thin tocooperate with the. insulating lm to modify the nature of an electricdischarge in a gas pocket bounded on one side by the filmed conductor inthe same way as does a relatively thick articially formed lm.

The semi-conductive inner portion of the interposed layer may, forexample, be a thin adherent lm of cuprous oxide of from 5-50 ,i thick.Where the cable conductor is of copper, such a iilm may be producedby'heating the conductor to a temperature of about 900 C. by drawing'itthrough a furnace to which air is admitted at such a rate that a film ofoxide of the required thickness is produced or by drawing the conductorthrough a bath of molten sodium nitrite at a temperature of about 530 C.at such a rate that a lm of the desired thickness is formed andsubsequently drawing it through a bath of hot water to remove any sodiumnitrite adhering to the oxide coated conductor. Alternatively thesemi-conductive inner portion of the composite layer may be a thinadherent iilm of graphite formed by coating the conductor with acolloidal suspension of graphite in water.

Of the numerous materials suitable for the formation of the outerportion of the interposed composite layer the following are mentioned byway of example: Insulating varnishes, gum dammar, and plastics, such asvinyl chloride, styrene and methacrylic ester.

A 'vitreous enamel which comprises an oxide of boron or one or moremetal borates or silicates or a mixture of one or more metal borateswith one or more metal silicates or a mixture of one or more metalborates and/or one or more metal lsilicates with one or more metaliluorides.

Siliceous material It will be appreciated that in selecting a materialfor the outer portion of the composite layer due regard should be paidto the nature of the material constituting the inner portion and to thepossibility of a chemical reaction between the two portions. Fiorinstance, where the inner semi-conductive portion is cuprous oxide itwill generally be inadvisable to employ a vitreous enamel for the outerportion unless precautions be taken to limit the reaction between theoxide layer and the vitreous materiall whilst the latter is in themolten state.

Where the-outer portion of the ci' mposite layer is a plastic, thisportion may be formed by spraying the semi-conductive coating of theconductor with a solution of the plastic in a. volatile solvent or byrunning the coated conductor through a bath of such a solution.

Where the semi-conductive coating on the conductor is graphite, thecoated conductor may be given an insulating coating of vitreous enamelof the type above described by drawing it through a bath of moltenconstituents of the enamel, the temperature of the bath and hence theviscosity of the enamel, and the rate of travel of the conductor beingadjusted to give the required thickness of coating. For example a bathof fused borax maintained at a temperature of Irom 850-950 C. may beused and, by adjusting the rate of travel of the conductor, a thinlayer, for instance, a layer of 5-50 y. thick may be obtained.Alternatively the constituents of the vitreous coating may be fused andthe product subsequently ground to a ne powder, preferably to a particlesize les than the thickness of the coating required, and applied to thegraphite coated conductor in the form of an aqueous paste which is thensubjected to heat, rst to' drive off the liquid and then to fuse theresidue and convert it into a coherent vitreous layer. For example, theconductor may be coated with a paste of finely ground glass resultingfrom the fusion of a mixture consisting of 100 parts by weight of boraxand l parts by weight of silica and the coated conductor drawn through afurnace to raise the coating to a temperature of about 900 C. to convertthe coating into a vitreous layer. Or the conductor with itssemiconductive coating of graphite may be electrolytically coated with athin lm of metal e. g. copper which is afterwards completely oxidised,and the oxide coated conductor be subjected to either of the aforesaidprocesses for obtaining a coherent vitreous layer, with the result thatthe oxide layer re-acts with and is dissolved in the vitreous layerwhilst it is in the molten state. For example, a conductor provided witha semiconductive layer of graphite and with an outer layer ofelectrolytically deposited metal may be heated to a temperature of about900 C. by drawing it through a furnace to which air is admitted at sucha rate that a film of oxide of about ,u thick-is formed. The oxidecoated conductor can then be drawn through a bath of molten borax,maintained at a temperature of from 850-950, at such a speed that theoxide layer is completely converted into copper borate with the resultthat a conductor having an inner coating of graphite and an outervitreous coating comprising a mixture of fused borax and copper borateis obtained. Yet another way in which the outer portion of the compositelayer may be formed is by electrophoretically depositing on the graphitecoated conductor a thin coating of the vitreous material in a powderedstate from a suspension thereof and subsequently converting this coatinginto a coherent layer by the application of heat. This last said methodof forming a layer of vitreous material on a substratum is described andclaimed in the specication of British Patent No. 484,777 dated September26th, 1936, and granted to N. V. Phillips Gloeilampenfabreken.

Where the outer portion of the composite layer is of silioeous materialthis portion may be formed by applying to the semi-conductive innerportion of the composite layer, a silica varnish comprising a solutionof silica, preferably a solution of silica in a non-aqueous solvent or amixture of such solvents or a mixture of such a solution and anundecomposed ester of silicic acid, and allowing the silioeous coatingto dry. Such solutions can be made, as described in the specification ofBritish Patent No. 290,717, dated February 16th, 1927, and granted to G.King and R. Threlfall, by reaction between an ester of silicic acid andwater in the presence of a solvent for the ester which is also adaptedto carry water either in solution or suspension or in the presence of amixture of such solvents. A silica varnish that has been found to bevery satisfactory is the partially decomposed solution of pure siliconesters described in Example l of the specication of the aforesaid patentand there referred to as the basis solution, diluted with an equalvolume of toluene. The conductor with its semi-conductive coating may begiven an adherent coating of silioeous material to form the outerportion of the composite protective layer by drawing it through a bathof the silica varnish at room temperature and at such a speed as toyield a film of the required thickness. For instance, with the basissolution referred to above a coating having a thickness when dry ofabout 40p is obtained by drawing through at a speed of the order of 1ft./sec. It appears important to prevent access of moisture to thecoated conductor during the setting of the lm in order to restrict lossof elasticity. Accordingly, it is preferred to dry the coated conductorin an oven and to take precautions to exclude water vapour and dusttherefrom both during the drying process and subsequently untilprotected by the superposed paper insulation. At a temperature of 110 C.a drying period of 45 minutes suffices.

In cases where stranded conductors are employed the conductor may begiven a smooth surface of non-re-entrant form, for instance, by applyingto the stranded conductor a metal tape wrapping, and the compositeprotective layer be applied to the outer surface of the metallicsmoothing layer. In cases where the application of the composite layerdoes not involve the use of high temperature the smoothing layer maycomprise a thin lead sheath. Alternatively the composite protectivelayer may be applied to each of the individual wires forming theoutermost layer or two outer layers of the stranded conductor prior totheir incorporation into the conductor.

The accompanying drawing shows two examples of high tension cableconstructed in accordance with the present invention. In the drawing.

Figure 1 is a cross-sectional view of a cable with one form of compositeprotective layer, and

Figure 2 is an enlarged cross-sectional view of the central part of acable with a second form of composite protective layer.

In each case the cable illustrated comprises a conductor i consisting ofa plurality of component wires 2 stranded together. The strandedconductor is insulated by an impregnated body of paper insulation 3. Onthe surface Tif the latter is a metallised layer 4 and the whole. isenclosed in an impervious metal sheath 5. In the form of constructionshown in Figure 1 the conductor is given a smooth surface ofnon-reentrant form by applying thereto a thin metal sheath B. On theouter surface of thissheath 6 is provided a composite protective layercomprising an inner portion 1 of semi-conductive material and an outerportion 8 of homogeneous insulating material. It is to be understoodthat the drawing is not to scale and that the thickness of each portionof the protective layer, which may, for instance, be about 40a, has beengreatly exaggerated in order that the composite nature of the layer maybe clearly shown. In the form of construction disclosed in Figure 2 ofthe drawing the metallic smoothing layer 8 is dispensed with and thecomposite protective layer is formed by providing each wire .2 of thetwo outer layers in the stranded conductor i with a thin inner coating 9of semi-conductive material and an outer coating I0 of homogeneousinsulating material. The thicknesses of these coatings 9 and l0 havealso been greatly exaggerated in order to show them more clearly.

What I claim as my invention is:

1. A high tension electric cable comprising a conductor, an impregnatedbody of fibrous insulation surrounding said conductor, an impervioussheath enclosing said body of insulation and a composite protectivelayer for said body of insulation interposed between the surface of saidconductor and the inner surface of said body, said composite protectivelayer comprising an inner portion consisting of an artificially formedfilm of semi-conductive material and an outer portion consisting ofhomogeneous insulating material impervious to the compound used toimpregnate the dielectric.

2. A high tension electric cable comprising a stranded conductor,consisting of a plurality of individual strands. a plurality ofcomposite prol tective layers, one for each of the outside strands bodyof fibrous insulation, said composite protecf of the said conductor, animpregnated body of iibrous insulation surrounding said strandedconductor, and an impervious sheath enclosing said body of insulation,said composite protective layers each comprising an inner portion ofsemiconductive material and an outer. portion of homogeneous insulatingmaterial impervious to the medium with which said brous insulation isimpregnated. A

3. A high tension electric cable comprising a stranded conductor, asmooth surfaced metallic layer surrounding said stranded conductor andelectrically connected thereto, a composite pro-r tective layer providedon the outer surface of said metallic layer, an impregnated body offibrous insulation surrounding said composite protective layer, and animpervious sheath enclosing said tive layer comprising an inner portionconsisting of an artificially formed film of semi-conductive materialand an outer portion consisting of homogeneous insulating materialimpervious to the medium with which said brous insulation is imfpregnated.

4. A high tension electric cable comprising a conductor, an impregnatedbody of fibrous insulation surrounding said conductor, an impervioussheath enclosing said body of insulation and a composite protectivelaver for said body of linsulation interposed between the surface ofsaid con-` ductor and the inner surface of said bOdy, Said compositeprotective layer comprising an inner portion consisting of a film ofsemi-conductive material having a thickness of about 5-50p and an outerportion consisting of homogeneous insu# lating material impervious tothe compound used to impregn'ate the dielectric.

5. A high tension electric cable comprising a stranded conductor,consisting of a plurality of individual strands, a plurality ofcomposite protective layersyone for each of the outside strands of thesaid conductor, an impregnated body of fibrous insulation surroundingsaid stranded conductor, and an impervious sheath enclosing said bodyVof insulation, said composite protective layers each comprising aninner portion consisting of a illm of semi-conductive material having athickness of from about 5-50a and an outer portion of homogeneousinsulating material impervious to the medium with vwhich said fibrousinsulation is impregnated. v

6. A high tension electric cable comprising a stranded conductor, asmooth surfaced metallic layer surrounding said stranded conductor andelectrically connected thereto, a composite protective layer provided onthe outer surface of said metallic layer, an impregnated body of fibrousinsulation surrounding said composite yprotective layer, and animpervious sheathV enclosing said body of brous insulation, saidcomposite protective layer comprising an inner portion consisting of afilm of semi-conductive materialhaving'a thickness of from about 5-50pand an outer portion consisting of homogeneous insulating materialimpervious to the rmedium with which said fibrous insulation isimpregnated.

ROBERT WINSTANLEY LUNT.

